Integrated iron reduction and smelting processes which use coal directly without the need for a separate coking step have been developed in recent years as alternatives to the widely-used blast furnace. These newer ironmaking processes utilize oxygen (typically greater than 95 vol % oxygen) rather than air as used in a blast furnace, and produce two to three times more medium-BTU gas per unit of iron production than a blast furnace. These processes include the COREX.RTM. process developed and marketed by Voest-Alpine Industrieanlagenbau and Deutsche Voest-Alpine Industrieanlagenbau, as well as other new or developing direct ironmaking methods such as the ROMELT, HiSMELT.RTM., DIOS, and the AISI In-Bath Smelting processes. Direct ironmaking processes have significant operating and environmental benefits over the conventional blast furnace process. In addition, direct ironmaking processes are well-suited for integration with combined cycle power generation systems which utilize the byproduct medium-BTU gas as primary fuel. The COREX.RTM. process, which is representative of these integrated reduction-smelting processes and which is currently in commercial use, is described in a paper entitled "Cogeneration with COREX.RTM." by M. Lemperle and D. Siuka presented at the 1991 AISE Annual Convention, Pittsburgh, Pa, September 1991.
Large volumes of oxygen are required for direct integrated ironmaking processes; the COREX.RTM. process for example requires approximately 0.6 to 1.0 ton of oxygen per ton of iron produced. This oxygen typically is produced in a cryogenic air separation process which usually is not integrated with the ironmaking process.
Oxygen can be recovered from air at high temperatures by metal oxide ceramic materials utilized in the form of selectively permeable nonporous ion transport membranes. An oxygen partial pressure differential or a voltage differential across the membrane causes oxygen ions to migrate through the membrane from the feed side to the permeate side where the ions recombine to form electrons and oxygen gas. An ion transport membrane of the pressure-driven type is defined herein as a mixed conductor membrane, in which the electrons simultaneously migrate through the membrane to preserve internal electrical neutrality. An ion transport membrane of the electrically-driven type is defined herein as a solid electrolyte membrane in which the electrons flow from the permeate side to the feed side of the membrane in an external circuit driven by a voltage differential. A comprehensive review of the characteristics and applications of ion transport membranes is given in a report entitled "Advanced Oxygen Separation Membranes" by J. D. Wright and R. J. Copeland, Report No. TDA-GRI-90/0303 prepared for the Gas Research Institute, September 1990.
In the recovery of oxygen from air at high temperatures (typically 700.degree. C. to 1100.degree. C.) using ion transport membranes, a significant amount of heat energy is available in the membrane permeate and non-permeate streams. Energy recovery and effective utilization thereof is possible by the integration of compressors, combustors, hot gas turbines, steam turbines, and heat exchangers with the mixed conductor membrane module. Such integrated systems are described in U.S. Pat. Nos. 4,545,787, 5,035,727, 5,118,395, 5,174,866, and U.S. Pat. No. 5,245,110. An article entitled "Separation of Oxygen by Using Zirconia Solid Electrolyte Membranes" by D. J. Clark et al in Gas Separation and Purification 1992, Vol. 6, No. 4, pp. 201-205 discloses an integrated coal gasification-gas turbine cogeneration system with recovery of oxygen for use in the gasifier. Membrane non-permeate is combusted with gas from the gasifier and passed to the gas turbine cogeneration system.
A combined cycle power generation system is a highly efficient system which utilizes a gas turbine to drive an electric generator, wherein heat is recovered from the turbine exhaust as steam which drives an additional electric generator. A description of typical combined cycle power generation systems is given in The Chemical Engineer, 28 Jan. 1993, pp. 17-20. The compressor, combustor, and expansion turbine are carefully designed and integrated to maximize the efficiency of each component and thus the efficiency of the integrated system. Preferably these systems are operated at steady-state design conditions, since significant deviations from these conditions will adversely affect system efficiency.
Integrated coal-based ironmaking processes, combined cycle power generation, and the production of oxygen by high-temperature ion transport membranes are well-suited for integration in highly-efficient systems for the production of iron, electric power, and oxygen. The invention disclosed below and described in the claims which follow advances the art and provides improved methods for the production of iron, electric power, and oxygen by means of integrated direct ironmaking, ion transport membrane, and combined cycle generation systems.